Acoustic attenuation device

ABSTRACT

An acoustic attenuation device including an elongated hollow body having an open upstream end taking the form of a head for coupling to a compressed air outlet, and a closed downstream end, the hollow body having a sidewall with a plurality of air exhaust openings arranged in the downstream part, where the hollow body contains at least one upstream buffer lining forming a longitudinal filter and at least one downstream filtering lining forming a sheath.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/911,294 filed on 11 Oct. 2007 under 35 U.S.C. 371 with respect toInternational Application No. PCT/FR2006/000724 filed on 3 Apr. 2006which claims priority to French Patent Application No. 05.03602 filed on12 Apr. 2005, all of which said applications are herein incorporated byreference in their entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an acoustic attenuation device designedto be connected to a compressed air outlet.

BRIEF DESCRIPTION OF RELATED ART

The abrupt release of a large quantity of compressed air may generate anoise of very high sound intensity, which can even become dangerous forpeople situated close by. Nevertheless, it is occasionally necessary topurge the air from a compressed air reservoir in this way so that anoperator can verify its state and make sure notably of its correctsealing, pressurization and air release. This is the case, for example,for the braking circuits of a train. Indeed, the braking of a train iscarried out by decreasing the pressure in the braking circuit, the shoesthen being forced against the wheels of the train in order to slow itdown. The pressurization, the sealing and the continuity of the pressurewithin the whole circuit are, amongst others, important parameters whichneed to be monitored.

The verification of the braking circuit of a train involves severalsteps performed by at least one operator. First of all, the operatorchecks the correct operation of each brake shoe, then a complete purgingof the air pipe of the braking circuit is carried out in order to verifyits continuity. Given that the pressure of the air in these pipes canreach as much as 9 bar, the intensity of the noise generated reachesaround 121 dB near to the compressed air outlet. This value is close tothe pain threshold and repeated exposure could lead to a hearinghandicap for the operator. It is not therefore acceptable to expose anoperator performing this type of check to such a sound intensity.

Noise attenuation devices have been developed in order to solve thisproblem and to allow an operator to complete his work safely. Anacoustic attenuation device is notably known comprising an elongatedhollow body designed to be connected to a compressed air outlet via afirst open end fitted with a coupling head and having air exhaustopenings arranged radially within a wall of the tube close to a secondclosed end. The acoustic attenuation is obtained by disposing inside thehollow body a knitted stainless steel cloth lining rolled-up on itselfand forming a buffer filtering the air. The effect of this is that theair flowing through the lining follows different paths, which slows thepropagation and the development of low- and high-pressure regions thatgenerate the noise.

Such a device allows the intensity of the noise to be reduced from 120dB to about 78 dB at the outlet of the compressed air pipe for an airpipe under a pressure of around 5 bar. This value of 78 dB may still bejudged to be too high by the operators and it is desirable to reduce it.It should however be noted that a check by an operator requires aminimum noise level to be conserved in order for the depressurization toremain audible. A standard dictates that the intensity of the noise ofthe air outlet from the pipe should not be below 70 dB. Furthermore,such a device reduces the speed of the air at the outlet and it is notdesirable to prolong the time required for a total purging of the pipetoo much. The same standard previously mentioned thus dictated that areservoir of 600 L of air at 5 bar should be able to be purged until thefinal pressure reaches 0.4 bar in less than about one minute.

BRIEF SUMMARY OF THE INVENTION

The goal of the present invention is to provide an improved devicemeeting the requirements of the standard while, at the same time,providing an enhanced acoustic attenuation, and for that purpose itconsists of an acoustic attenuation device comprising an elongatedhollow body having an open upstream end taking the form of a head forcoupling to a compressed air outlet, and a closed downstream end, saidhollow body having a sidewall with a plurality of air exhaust openingsarranged in the downstream part, characterized in that the hollow bodycontains at least one upstream buffer lining forming a longitudinalfilter and at least one downstream filtering lining forming a sheath.

Thus, by providing at least one downstream lining functioning as radialfilter, it has been observed that a device according to the inventionallowed the noise generated to be attenuated more efficiently.

According to a first variant embodiment, the upstream lining is formedby rolling up on itself a filtering mesh formed of a suitable material.

According to a second variant embodiment, the upstream lining takes theform of a compartment containing lining elements formed from a looselypacked suitable material.

The materials used for the upstream lining could notably be based onstainless steel, synthetic materials or composites. Preferably, aknitted stainless steel cloth will be used, rolled up on itself orloosely packed, since this material is particularly satisfactorilyresistant to the longitudinal compression that the flow of compressedair causes.

Advantageously, the downstream lining is disposed around an open mandreldesigned to hold it. The open mandrel could, for example, be formed by aspring of round-section wire, thus allowing its resistance to the airintended to flow through the device according to the invention to bereduced.

Preferably, the downstream lining is formed by winding of at least oneband of at least one suitable material around the mandrel.

According to one preferred embodiment, the downstream lining is composedof at least one sheath formed of a suitable filtering materialsurrounded by a retaining jacket under tension. Advantageously, thefiltering sheath and the retaining jacket under tension are formed frombands of the same width wound one over the other around the mandrel.Again advantageously, the band designed to form the retaining jacketunder tension has a greater length than the filtering band. Thus, theband designed to form the retaining jacket under tension is wound oncearound the mandrel before winding it with the filtering band.

Preferably, the retaining jacket under tension is made of stainlesssteel cloth.

In an advantageous manner, the downstream lining comprises at least onecomposite material. Preferably, the composite material is a non-wovenfiber glass material. This material is particularly recognized for itsacoustic insulation properties.

Advantageously, a multi-perforated washer is disposed at the inlet ofthe upstream lining. Thus, the flow of air passing through the device ismore efficiently distributed over the whole cross-section of theupstream lining.

In an even more advantageous manner, a centrally-perforated washer isdisposed between the upstream lining and the downstream lining. Thus,the flow of air is channeled at the outlet of the upstream lining andcentered on the axis of the tube of the downstream lining.

It should be noted that the present invention is not limited to a devicewith two linings and that it is perfectly possible to dispose severallinings forming a buffer and/or several linings forming a sheath.

BRIEF DESCRIPTION OF THE DRAWINGS

The implementation of the invention will be better understood with theaid of the detailed description which is presented hereinbelow withreference to the appended drawings, in which:

FIG. 1 is a schematic longitudinal cross-sectional representation of anacoustic device according to the invention.

FIG. 2 is a cross-sectional view along the line II-II in FIG. 1.

FIG. 3 is a cross-sectional view along the line III-III in FIG. 1.

FIG. 4 is a cross-sectional view along the line IV-IV in FIG. 1.

FIG. 5 is a cross-sectional view along the line V-V in FIG. 1.

FIG. 6 is a representation of the disposition of two layers of materialsbefore winding around a mandrel.

DETAILED DESCRIPTION OF THE INVENTION

An acoustic attenuation device, such as is shown in cross section inFIG. 1, comprises an elongated hollow body 1 formed from a cylindricaltube 2, a coupling head 4 and a base 5.

The tube 2 has air exhaust openings 6 arranged radially in a sidewall 9of the downstream part. Furthermore, the tube 2 is preferably made of amaterial that is light but capable of resisting high pressures. Thismaterial could, for example, be a composite material molded under vacuumthen fired. Advantageously, the sidewall 9 of the tube 2 comprises onits upstream part an anti-slip coating so as to facilitate handling andmaneuvering of the device.

The coupling head 4 is designed to allow the connection of the deviceonto a compressed air outlet (not shown) of a conduit of a brakingcircuit of a train, and has corresponding standard dimensions.Furthermore, the coupling head 4 comprises a fixing lug 10 allowing thedevice to be suspended from a belt for example. The coupling head 4,generally made from gray cast steel, could advantageously be made from alighter material, such as cast aluminum. The coupling head 4 is rigidlyfixed, for example by bonding, into the upstream end 7 of the tube 2.

With the coupling head 4 thus fixed, the lining of the tube 2 isinstalled. The latter firstly comprises a multi-perforated washer 11having a diameter substantially equal to the diameter of the tube 2,such as is shown in FIG. 2, pushing up against the coupling head 4 andagainst which an upstream lining 12 forming a buffer presses, as shownin FIG. 3. This upstream lining 12 is formed, for example, from aknitted stainless steel cloth rolled up on itself.

It should be noted that other materials may be used for the upstreamlining 12, for example, synthetic materials, composites or even naturalmaterials. This upstream lining 12 may furthermore be formed usingmaterials taken in the form of a filtering mesh or loosely packed in acompartment, one wall of which may be formed by the multi-perforatedwasher 11.

The upstream lining 12 is closed by a centrally-perforated washer 13having a diameter substantially equal to the diameter of the tube 2,such as is shown in FIG. 4, and pushing against the upstream lining 12.This centrally-perforated washer 13 is followed by a filteringdownstream lining 14 forming a sheath, shown in FIG. 5. It should benoted that the centrally-perforated washer 13 could form a second wallof the compartment in the case where the upstream lining 12 is formedfrom loosely packed elements.

The downstream lining 14 is disposed around an open mandrel 15 formed bya spring of round-section wire and designed to be used as a support forthe materials forming the downstream lining 14. The use of around-section wire spring allows a minimal resistance to be offered tothe air passing through it. More precisely, the downstream lining 14comprises, on the one hand, a retaining jacket under tension formed froma band of stainless steel cloth 16 and, on the other hand, a band ofcomposite material 17, for example non-woven fiber glass. The band ofstainless steel cloth 16 and the band of composite material 17 are woundtogether around the open mandrel 15. Such a winding process results inalternating layers of stainless steel cloth 16 and of composite material17. In the case where the composite material 17 is a non-woven fiberglass for example, the fibers are thus held by the stainless steel cloth16 and the composite material 17 will not be degraded by the effect ofthe passage of compressed air. Each layer of stainless steel cloth 16thus forms a pressure-retaining sheath surrounding a layer of filteringmaterial.

The band of stainless steel cloth 16 and the band of composite material17 have identical widths and are wound together around the mandrel 15.However, the band of stainless steel cloth 16 is longer than the band ofcomposite material 17, the difference in length corresponding to a firstturn of the band of stainless steel cloth 16 around the mandrel 15. Thedisposition of the two bands before winding is shown in FIG. 6.

It should be noted that the exhaust openings 6 in the downstream part ofthe tube 2 are distributed along the downstream lining 14. Moreprecisely, the tube 2 has three circular regions each comprising fourexhaust openings 6 uniformly distributed around the circumference of thetube 2.

Once the lining has thus been disposed inside the tube 2, the base 5 isthen thermo-bonded to the tube 2 at a downstream end 8 so as to closeoff said downstream end 8. The base 5 will advantageously be made ofcast aluminum owing to the pressure forces to which it will besubjected. It should however be noted that the tube 2 and the base 5 maybe formed as a single component, the lining then being introduced intothe tube 2 via the upstream end that is subsequently closed off by thecoupling head 4.

An operator performing a check of the train braking circuit will proceedin the following manner.

First of all, the operator connects the device to the air outlet that hewishes to monitor by means of the coupling head 4.

The stop-valve is then opened and the purging of the pipe started. Thecompressed air flows through the coupling head 4 then enters at highspeed into the whole assembly of the hollow body 1.

This air firstly flows through the multi-perforated washer 11 whichdivides it into a plurality of flow streams distributed over the wholecross section of the tube 2 before penetrating into the inside of theupstream lining 12 which it crosses longitudinally. At the exit of theupstream lining 12, the air flow passes through the centrally-perforatedwasher 13 which channels this air flow inside the sheath formed by thedownstream lining 14. Finally, the air flows radially through the openmandrel 15 and the downstream lining 14 before exiting from the devicevia the exhaust openings 6.

Thus, by flowing through the suitably-designed linings, the latter breakup the regions of high and low pressure of the propagating air, whichreduces the noise generated.

Experiments have demonstrated that the 120 dB produced by adepressurization without acoustic attenuation device of a compressed airpipe under 5 bar fall to 74 dB by using the attenuation device accordingto the invention. This represents a gain of 3 dB, or a sound intensityreduced by a factor of two with respect to the device known from theprior art, and adheres to the minimum audibility threshold of 70 dBfixed by the standard.

It should also be understood that the invention is not limited to asingle lining forming a buffer and that it is of course possible toprovide a plurality of stages of each type, buffer or sheath.

Although the invention has been described in association with particularexemplary embodiments, it will be clearly understood that it is not inany way limited and that it encompasses all the technical equivalents ofthe means described, including their combinations if these fall withinthe scope of the invention.

1. An acoustic attenuation device comprising an elongated hollow body having an open upstream end taking the form of a head for coupling to a compressed air outlet, and a closed downstream end, said hollow body having a sidewall with a plurality of air exhaust openings arranged in the downstream part, wherein the hollow body contains at least one upstream buffer lining forming a longitudinal filter and at least one downstream filtering lining forming a sheath.
 2. The device as claimed in claim 1, wherein the upstream lining is formed by rolling up on itself a filtering mesh formed of a suitable material.
 3. The device as claimed in claim 1, wherein the upstream lining takes the form of a compartment containing lining elements formed from a loosely packed suitable material.
 4. The device as claimed in claim 1, wherein the downstream lining is disposed around an open mandrel designed to hold it.
 5. The device as claimed in claim 4, wherein the downstream lining is formed by winding of at least one band of at least one suitable material (around the mandrel.
 6. The device as claimed in claim 4, wherein the downstream lining is composed of at least one sheath formed of a suitable filtering material surrounded by a retaining jacket under tension.
 7. The device as claimed in claim 6, wherein the filtering sheath and the retaining jacket under tension are formed from bands of the same width wound one over the other around the mandrel.
 8. The device as claimed in claim 7, wherein the band designed to form the retaining jacket under tension has a greater length than the filtering band.
 9. The device as claimed in claim 6, characterized in that the retaining jacket under tension is made of stainless steel cloth.
 10. The device as claimed in claim 1, wherein the downstream lining comprises at least one composite material.
 11. The device as claimed in claim 10, wherein the composite material is a non-woven fiber glass material.
 12. The device as claimed in claim 1, wherein a multi-perforated washer is disposed at an inlet of the upstream lining.
 13. The device as claimed in claim 1, wherein a centrally-perforated washer is disposed between the upstream lining and the downstream lining. 